Astronomers have found one of the oldest known stars. And it’s old: 13.2 billion years old. The Sun, by comparison, is 4.6 billion years old. A veritable baby!

Dating a star is hard (in both senses of both words), but there are telltale signs of age. One thing you can do is look for stars that are almost entirely hydrogen and helium, and there’s a good chance they’ll be really old. In the early Universe, those two elements were pretty much all there was. When the first generation of stars formed, they created heavier elements like carbon, oxygen, and even much heavier elements like iron and so on up the periodic table. When they exploded they scattered those heavy elements around them, which eventually became part of dust and gas clouds which formed the next generation of stars. So younger stars tend to have more heavy elements than older stars. If you’re looking for old stars, you look for ones with higher proportions of light elements. They may still have heavier elements in them, just not as much as younger stars, which formed from clouds which had successive generations of exploding stars seed them with ever more heavier elements.

So they turned to a natural clock that appears in stars: radioactive elements. In this case, specifically uranium, europium, thorium, and osmium. By measuring how much of these elements are in the star and knowing their decay rates, it’s possible to determine how long they’ve been sitting in that star. Since the elements were created shortly before the star was born, this gives you a pretty good estimate for the age of the star!

How do you do that? Well, you take a spectrum. You spread the colors of the light out, and measure how bright each color is. Different substances emit and absorb light at very specific and different colors. I’m simplifying, of course, but that’s the basic idea. One clear indicator of oxygen in a gas cloud, for example, is bright emission of light in the green part of the spectrum, at a wavelength of 501 nanometers.

So uranium has its own set of colors. If you can find them in the spectrum, you can figure out how much uranium there is. And that’s what Dr. Frebel and her team did. They pointed their spectrograph at the star HE 1523-0901, and this is what they got:

The wavelength (color) on the x-axis is in the blue end of the spectrum, and the y-axis is brightness. The dots are the measured spectrum. All the dips in the brightness represent light being absorbed by different things. In this one you can see iron (Fe), magnesium (Mg), and even, wow, neodymium (Nd). The dip at 3859.6 Angstroms is from uranium absorption.

You can use physics and math and predict how much uranium you’d expect to see based on the age of the star and the amounts of other elements. If there were no uranium at all you’d see the spectrum where the solid blue line is. But the amount of uranium indicates the star in question in 13.2 billion years old. This was substantiated by other dating methods as well (including measuring europium, osmium, and iridium).

Wow. That means this star formed just 500 million years after the Big Bang. The very first stars formed about 400 million years after the BB, so this star coalesced only 100 million years after those stars first did– it may be from the very first stars in that second generation. Think of the changes since then! The Universe was far smaller and warmer when that star was first born. The Milky Way was brand new, and the birth of the Sun was still 9 billion years in the future. This star was old when the Earth was born.

That’s amazing work. I wonder what else this star can tell us? The abundances of the other elements may tell us about those very first stars, the ones that blew up and created the first heavy elements. I bet that eventually, the abundances in this star will tell astronomers more about how massive and how hot the first stars were. One the coolest things about science is that all the pieces fit together. They have to. Science is how we figure out reality, and I’m pretty sure reality works.

Comments (23)

Links to this Post

I know there are stable isotopes of both osmium and iridium. How do you tell whether you are dealing with radioactive osmium and iridium or stable versions? They are dealing with the near ultraviolet spectrum at least in that plot, and radiation at those frequencies tends to interact with electrons and not nuclei. I would suspect it would be difficult, if not impossible, to differentiate isotopes based on a near-UV spectrum.

Yeah, you have to remind them to wear panties with their miniskirts so they don’t flash the paparazzi, make sure their friends aren’t taking pictures of them snorting cocaine, and put up with their obsession with adopting children from third world countries.

Basic question you say “Since the elements [uranium, europium, thorium, and osmium] were created shortly before the star was born.” I’m not familiar with how those elements were formed shortly before the star was born. I though I’d heard that all elements bigger than iron were formed by supernova or something. Can you tell me what don’t I know here?

TheBlackCat, you can know if there are isotopes of a given element by looking at the line profile; since its wavelength is slightly different, you will find an assymetric profile instead of a gaussian one.

AgnosticOracle, those elements are indeed thought to have been formed in one (or maybe more) earlier generation(s) of stars. If I understand it correctly So this wouldn’t be one of the really REALLY first stars, but a very old one nonetheless.

BA, can you explain in laymen’s terms why the assumption that the star must have formed soon after its predecessor went boom is valid? Isn’t it possible that the ejecta remained in a gascloud only to be perturbed by another process billions of years later or is that so unlikely that it can be disregarded?

Also, it seems they assume an initial amount of the isotopes. Is there an easy way to explain that part of the equation?

Don’t get me wrong, I want this star to be old. I want it to be the grandmother of all stars, able to tell us about the good old days, before all these youngster and their rock ‘n roll lifestyles. But those questions keep nagging me every article I read about his discovery :/

The elements Iridium, Osmium, Europium, Uranium, and Thorium would have all had to have been formed in a preceeding supernova. They cannot date this star without knowing the ratios of those elements in the original nebula formed after the supernova.

You can use physics and math and predict how much uranium youâ€™d expect to see based on the age of the star and the amounts of other elements.

So they based their calculations of the age of this star on their plug-in assumptions of the age of the star? That’s like a dictionary definition of a zebra that says “an animal that looks and acts like a zebra”. It is begging the question.

Ed Minchau said: The elements Iridium, Osmium, Europium, Uranium, and Thorium would have all had to have been formed in a preceeding supernova. They cannot date this star without knowing the ratios of those elements in the original nebula formed after the supernova.

I stand to be corrected, but I think the idea is that stellar fusion from hydrogen and helium produces elements in pretty fixed ratios, and each reincarnation changes those ratios based on what they can fuse into.

Ed Minchau: So they based their calculations of the age of this star on their plug-in assumptions of the age of the star? Thatâ€™s like a dictionary definition of a zebra that says â€œan animal that looks and acts like a zebraâ€. It is begging the question.

I’m interpreting it that the original sentence was badly phrased. The decay of uranium takes place at a (more or less) fixed rate, and it then becomes some other element with a different spectrum. Other elements in the chart do not decay and will remain at original levels. So looking at the overall chart, having one from column A and two from column B should also indicate a certain number from column Uranium, at the time the parent star went blooey. But uranium disappears over time, so you look at the amount that has, in effect, disappeared and you know how much time has passed. This can potentially be confirmed if you also get a spectral reading off of the elements that the uranium would have decayed into. I suspect that the sentence should have read more like, “You can use the amount of uranium to judge the age of the star.”

Uranium 238 has a half-life of 4.46 billion years, so this allows for using the decay rate with a certain degree of accuracy over very long periods. But again, somebody who has more eddikashun than me should feel free to jump in.

Now, my question is, just how far away is this star? How old is that light that we’re making the measurements from when it reaches us? And what kind of conditions are necessary to have a star live that long in the first place?

I stand to be corrected, but I think the idea is that stellar fusion from hydrogen and helium produces elements in pretty fixed ratios, and each reincarnation changes those ratios based on what they can fuse into.

That’s my understanding, too. The link BA gives says that the Europium, Osmium, and Iridium were ‘anchor’ elements, so I think they must have had a theoretical prediction of the amounts of these elements relative to the radioactive ones (and I think these predictions can be validated for more recent explosions from looking at pre-solar grains in primitive meteorites). The current levels of the radioactive elements give the age. My very vague recollection is that Iridium only has one stable isotope so they may not have needed to discriminate between isotopes in the measurements.

I guess a hundred million years still gives time for several generations of supernovae in theory – I don’t know how that affects the calculations.

This is very interesting. Some of the comments have addressed some of my questions, but I would feel reassured if BA or someone more knowledgeable would confirm or clarify. I wondered how they would know the original ratios of these elements in the prestellar cloud (it seemed to me that they would need to know this). Just Al’s suggestion that supernovae produce very heavy elements in predictable ratios had occured to me and seems plausible. Although I don’t believe that we have ever observed the explosion of a first generation star, perhaps we combine our observations of supernovae of stars born later with our knowledge of nucleosynthesis. Still, the calculation might seem “iffy” for one cosmic clock. http://mcdonaldobservatory.org/news/releases/2007/0510.html
refers to “six cosmic clocks.” I supposed that the number six came from pairing each of uranium and thorium with each of europium, osmium, and iridium. I guess if each of the six clocks gave a very similar result, the whatever assumptions were made about the original ratios would be strengthenede tremendously.

I always wonder about the error bars. Now I see that news@nature.com (sorry, I’m not good at providing links) reports the estimate as 13.2 billion years PLUS OR MINUS 2 BILLION YEARS. I guess that leaves room for several generations of supernovae and somewhat looser assumptions than I had supposed.

Found the paper at arxiv, I’ll blog it once my conference talk is done.

Basically-
1. assume initial Eu, Ir, Os, Th, U ratios at production (she uses 4 or 5 different models)
2. Assume star forms immediatly after heavy elements are produced (this is what happened in our system- why not extrapolate to the universe)
3. Assume the difference between measured and expected U and Th is due to radioactive decay
4. use decay constants to determine age.

She does all this, and ends up with an answer of 13.2 +/- 2.5 Ga.

Most of the error seems to be from the initial composition assumptions adn disagreement between the stable Os, Ir, or Eu, and not the analysis.

The decay product of Th and U, Pb, cannot be measured, as there is a spectral interference.

Thanx. A lot of that was in accord with my suppositions. I had not considered the possibility of her using more than 1 model, although it now seems obviously proper technique to consider several. I had wondered why no one mentioned the lead.

It must be quite difficult to find suitable candidates for this sort of analysis.

Brian:
Evidently one of the biggest problems is that the best U peak overlaps the C-N peak. So you need either very low C,N stars, or hot stars (to break the molecular bond). But of course, hot stars have shorter life expectancy. As a result, I think the two stars that they looked at are giant branch stars- they are not main sequence red dwarves.